A national memorial service for the Australian victims of the downed flight MH17 will be held in Melbourne next Thursday, coinciding with a national day of mourning.

Tony Abbott announced the memorial plans while admitting his frustration at the inability of the multinational police team, including unarmed Australian federal police, to enter the wreckage site in eastern Ukraine to recover bodies.

The prime minister said the Ukrainian government was trying to “reassert its sovereignty over the whole country” while Russian-backed separatists were “determined to stop that”, but the police team would “not readily be deterred”.

Australia’s foreign affairs minister, Julie Bishop, was more forthright in assessing the role of Moscow, saying she feared that Russia was “actively undermining” the process of securing safe access amid reports that the remains of up to 80 victims may still be in the war-torn area.

“The Ukrainian government has a military objective and that is to retain its sovereignty and obviously take back territory that was filled by the separatists, but they have been offering us a lot of support for a humanitarian corridor and a ceasefire,” Bishop told the ABC on Thursday.

“But we sent out reconnoitres in the morning; we send out a team in advance and they get halfway down the road, maybe a quarter of the way down the road, sometimes three-quarters of the way down the road – we tried this three days in a row – and then the shelling starts and while I can’t point the finger at who starts the shelling, we get the absolute assurances from the Ukraine government that it’s not them.”

Flags on all Australian government buildings will be flown at half-mast on Thursday, and an interdenominational memorial service will be held at St Patrick’s cathedral in Melbourne from 10.30am. The ceremony will be open to the public.

“The service will honour those who have lost their lives and seek to support those who have been bereaved,” Abbott said in a statement.

“All Australians mourn the loss of flight MH17. There were 298 innocent people on this aircraft and their deaths offend our sense of justice. We grieve for all, but particularly for the 38 men, women and children who called Australia home. Our hearts go out to all their families. We will support them through the difficult times ahead – 23 million Australians share the sadness of those who mourn.”

Abbott said the service would be held in Melbourne because 16 of the victims came from Victoria.

In a radio interview, the prime minister said the government was determined to do what it could to recover the remains of victims “and to get them justice”.

“We’re not going to take unnecessary risks but we are prepared to take reasonable risks to do an important job, because let’s never forget … 38 innocent Australians have been murdered; 38 innocent Australians have been shot down out of the sky; 298 innocent people have been shot down out of the sky,” Abbott told 3AW on Thursday.

Abbott said nothing had emerged to cast doubt on his initial assertions that the Malaysia Airlines plane had been shot down by Russian-backed rebels, and in a comment directed at Russia suggested “it would be much better if we didn’t have large countries interfering in the affairs of small ones”.

But the prime minister said he would like the Russian president, Vladimir Putin, to be in a position to continue to attend the economically focused G20 meeting in Brisbane in November.

“I would like it to go ahead as an opportunity for the leaders of the largest and most representative economies to sit down constructively and collegially to work out what we can’t do, singularly and together, to make a more prosperous world, to make a more financially secure world, and it would be better if Russia was there than absent, but I accept that that’s not the only consideration at play here,” he said.

Russia’s charge d’affaires in Canberra, Nikolay Nozdrev, told Fairfax Media the Ukrainian government was responsible for frustrating the police team’s efforts to enter the site by launching a “major offensive against the Donetsk people’s republic”.

In the interview, Nozdrev said Russia believed a new United Nations security council resolution was needed to provide a mandate for “armed teams” to access the wreckage site.

Abbott said the existing security council resolution, resolution 2166, provided ‘all the authority needed for the unarmed police mission”.

“We don’t need a further resolution and I’m always anxious with further resolutions that there might be ulterior motives involved, that it might all be about trying to secure some kind of an advantage in the games being played in eastern Europe,” Abbott said.

Those wishing to attend next Thursday’s memorial service have been asked to contact a ceremonial and protocol officer on 02 6271 5991 or 02 6271 5209 or via email at MemorialService@pmc.gov.au by noon on 6 August.